1. Field of the Invention
The present invention relates to an information reproduction method and an information reproduction apparatus, and more particularly to an information reproduction method and an information reproduction apparatus to be applied to an information recording medium such as an optical disk, a magneto-optical disk, a compact disk (CD), a compact disc recordable (CD-R) and the like.
2. Description of Related Art
There is conventionally known a Partial Response Maximum Likelihood (PRML) system as a signal processing system in an optical information reproduction apparatus such as an optical disk drive. In the PRML system, the waveform equalization of a reproducing signal is performed by a partial response (PR) system according to the characteristics of a recording and reproducing system, and decoding processing is performed by maximum likelihood decoding such as Viterbi decoding. Thereby, even if the reproducing signal is one having large intersymbol interference, data having a low error rate can be obtained. For calculating the likelihood of the Viterbi decoding, an ideal sampled value determined by the PR system used for the waveform equalization processing is used as the value of a discrimination point.
However, in an optical disk, the level of a reproducing signal is different from the discrimination point, which is uniquely determined according to the PR system, owing to nonlinear distortion such as the phenomenon called as asymmetry such that the eye pattern of a reproducing signal is asymmetry. The difference between the reproducing signal level and the discrimination point is a primary factor of the deterioration of the decoding performance of the Viterbi decoding.
To that problem, Japanese Patent Application Laid-Open No. 2001-250334 discloses an information reproduction apparatus and an information reproduction method which enable the discrimination point of the Viterbi decoding to be based on an actual reproduction waveform by setting the discrimination point on the basis of the maximum value and the minimum value of the amplitude of the shortest record information, the maximum value and the minimum value of the envelope of a reproducing signal, and the center value at the time of the binarization of the reproducing signal.
In the prior art, a discrimination point according to the asymmetry of a reproducing signal is set, and then the Viterbi decoding is performed. However, in the case were intervals between adjoining discrimination points become uneven owing to the correspondence to the asymmetry, the values of likelihood calculated at each time are dispersed, and consequently a problem in which the likelihood of the series of transitions at each time in the Viterbi decoding cannot be accurately judged is caused. The problem is described by referring to FIG. 9.
FIG. 9 shows values of discrimination points of the Viterbi decoding corresponding to PR(1, 2, 2, 1) which values are set according to asymmetry. The discrimination points in case of existing no asymmetry are seven of 0 to 6, but discrimination points g0 to g6 are set at uneven intervals correspondingly to nonlinear distortion such as the asymmetry in FIG. 9.
Hereupon, in the case where the value of a reproducing signal is 4.6 (B in FIG. 9), the likelihood between the value and discrimination points g4 and g5 are severally as follows.B(g4)=(4.6−g4)2=0.16B(g5)=(4.6−g5)2=0.16
Moreover, in the case where a value 1.6 (A in FIG. 9) of a reproducing signal, the likelihood between the value and the discrimination points g1 and g2 are severally as follows.B(g1)=(1.6−g1)2=0.36B(g2)=(1.6−g2)2=0.36
As shown in FIG. 9, the reproducing signal 4.6 is at a position distant from the discrimination points g4 and g5 equally, and the reproducing signal 1.6 is at a position distant from the discrimination points g1 and g2 equally. Consequently, it is suitable to consider that the probability that the reproducing signal 4.6 is the discrimination points g4 or g5 and the probability that the reproducing signal 1.6 is the discrimination points g1 or g2 are equal, but the above-mentioned branch metrics show different values from each other, 0.16 and 0.36. Thus, the probability disperses dependently on the relation between the reproducing signal and the discrimination point, and then the evaluation of a path metric at the time of the determination of the decoded data of the Viterbi decoding is not performed accurately. Consequently, the decoding property is lowered.